This invention relates to treatment of wastewaters containing BOD, phosphorus and nitrogen, such as municipal sewage, industrial wastewaters and the like by an activated sludge process. More particularly, the invention relates to a process whereby a wastewater influent is mixed with a denitrified-mixed liquor under anaerobic conditions before being mixed with recycled activated sludge in subsequent aeration zones.
Activated sludge processes have been used to remove biological oxygen demand (BOD) from municipal sewage, industrial wastewaters and the like. In such a process, a wastewater influent is mixed with a microorganism-containing recycled biomass or activated sludge in an initial contact zone to form a mixed liquor. At some point in the process, the mixed liquor is aerated with sufficient oxygen to grow and maintain a satisfactory population of microorganisms which sorb, assimilate and metabolize the BOD of the wastewater.
In the activated sludge process disclosed in U.S. Pat. No. 3,964,998, wastewater and recycled activated sludge are mixed with mechanical stirrers in a first stage which is operated under anoxic conditions. The mixed liquor is subsequently aerated in a second stage, subjected to anoxic conditions in a third stage, aerated in a fourth stage and then clarified to separate an activated sludge.
In another process, wastewater and recycled activated sludge are mixed and circulated around a plurality of concentric, annular basins or channels by a plurality of surface aeration discs or other mechanical surface aeration devices which chum oxygen into the upper surface of the mixed liquor and provide sufficient agitation to prevent settling. The mixed liquor flows from one channel to the next and finally is introduced into a clarifier to separate an activated sludge. The channels can be operated as a series of complete mix reactors so that the dissolved oxygen content in the first channel in which the wastewater and recycled activated sludge is initially mixed is about zero or less and the dissolved oxygen content is subsequently increased as the mixed liquor moves from one channel to the next.
Other activated sludge wastewater treatment processes are disclosed in U.S. Pat. Nos. 3,764,523, 3,939,068, 3,953,327 and 3,994,802.
The invention provides a process for treating BOD, nitrogen and phosphorus containing wastewater. The process comprises introducing wastewater influent into an anaerobic zone having activated sludge and mixing the wastewater influent with the activated sludge in the anaerobic zone to form a mixed liquor. The mixed liquor is introduced into an oxygen-deficit aeration zone, and denitrified mixed liquor from the oxygen-deficit aeration zone is recycled to the anaerobic zone for mixing therein with wastewater. The process further comprises transferring the mixed liquor from the oxygen-deficit aeration zone to an oxygen-surplus aeration zone, transferring a portion of the mixed liquor from the oxygen-surplus aeration zone to a settling zone wherein a supernatant is separated from settled sludge and recycling at least a portion of the settled sludge to the oxygen-deficit aeration zone as recycled activated sludge.
In another aspect of the invention, a process for treating BOD-containing wastewater using a system employing a plurality of concentric, annular zones is provided. The process comprises introducing wastewater influent into an anaerobic zone having activated sludge and mixing the wastewater influent with the activated sludge in the anaerobic zone to form a mixed liquor. The mixed liquor is introduced into a first aeration zone maintained under conditions which produce a complete mix reaction and provide insufficient oxygen to meet, the biological oxygen demand of the resulting mixed liquor. Denitrified-mixed liquor from the first aeration zone is recycled to the anaerobic zone for mixing therein with wastewater. The process further comprises transferring the mixed liquor from the first aeration zone to a subsequent aeration zone maintained under conditions which produce a complete mix reaction and provide sufficient oxygen to produce an overall dissolved content of at least 0.5 mg/L, transferring the mixed liquor from the subsequent aeration zone to a settling zone wherein a supernatant is separated from settled sludge and recycling at least a portion of the settled sludge to the first aeration zone as recycled activated sludge.
In another aspect of the invention, another process for treating BOD, nitrogen and phosphorus containing wastewater is provided. The process comprises introducing wastewater influent into an anaerobic zone having activated sludge, mixing the wastewater influent with the activated sludge to form a mixed liquor and introducing the mixed liquor into an aerated anoxic zone promoting simultaneous nitrification and denitrification. Denitrified mixed liquor from the aerated anoxic zone is recycled to the anaerobic zone for mixing therein with wastewater. The mixed liquor is transferred to a subsequent aeration zone under conditions which produce a complete mix reaction and provide sufficient oxygen to product an overall dissolved oxygen content of at least 0.5 mg/L, and then transferred from the subsequent aeration zone to a settling zone wherein supernatant is separated from settled sludge. At least a portion of the settled sludge is recycled to the aerated anoxic zone.